1. Field of the Invention
The present invention relates to dipole ring magnetic field generating devices.
2. Description of Related Art
A dipole ring magnetic field generating device is a magnetic field generating device comprising a plurality of magnet elements arranged in a ring shape such that a magnetization direction of the magnet elements undergoes one rotation over half a circumference of the ring, wherein the magnet elements generate a substantially unidirectional magnetic field in a space within the ring, as will be explained in more detailed below, and preferably, the magnet elements have the substantially the same maximum energy product. Thus arranging the magnets regularly along the circumference of a circle, a magnetic field that is substantially unidirectional and moreover preferably of substantially uniform strength can be generated in the space within the ring. Such dipole ring magnetic field generating devices are widely used, for example, for magnetic resonance imaging (MRI) devices and semiconductor element fabrication processes, as well as for uniform magnetic field generating means for fundamental research. Conventionally, normal electromagnets or superconducting electromagnets are used as means for generating a uniaxial uniform magnetic field. However, due to recent research in high-performance rare earth permanent magnets, rare earth permanent magnets (also referred to simply as “permanent magnets” in the following) are becoming mainstream in uniform magnetic field generating devices for weak magnetic fields of not more than, for example, 1 T (tesla: kg·s−2·A−1).
Referring to FIGS. 7 and 8, the following is an explanation of a conventional dipole ring magnetic field generating device and the constituent magnets used to configure this device.
FIG. 7 is a cross-sectional view of a dipole ring magnetic field generating device 1 taken from a direction perpendicular to the radial direction. As shown in FIG. 7, the dipole ring magnetic field generating device 1 includes a plurality of constituent magnets 101 to 124 that are arranged in a ring, and preferably, their outer circumference is enclosed by a ring-shaped outer yoke 2. Preferably, the individual constituent magnets 101 to 124 are magnetized in the directions given by the Equations (1) and (2), as will be explained in more detail below, and constituent magnets corresponding to opposite poles as seen from the center axis (for example, the constituent magnets 101 and 113) are magnetized with an angular difference of 180°, such that these constituent magnets have the same magnetization direction when they are arranged in a ring. Thus, when the constituent magnets 101 to 124 are arranged in a ring, the magnetization direction of the constituent magnets undergoes one rotation over half a circumference of the ring. With this configuration, a magnetic field is generated in the space within the ring of the dipole ring magnetic field generating device 1, which is substantially unidirectional and preferably of substantially uniform strength. It should be noted that it is preferable that the constituent magnets 101 to 124 have a magnetic field of the same strength as that unidirectional magnetic field.
For the constituent magnets 101 to 124, it is possible to use substantially trapezoidal or fan-shaped Nd—Fe—B, Sm—Co or Sm—N—Fe permanent magnets or the like. For the outer portion, it is possible to use a ring-shaped ferromagnetic or non-magnetic material as the outer yoke 2. In particular when a ferromagnetic material is used for the outer yoke 2, the magnetic efficiency is increased, if only slightly. Furthermore, the constituent magnets may be partitioned into 4 to 60 sections. In particular, in view of the magnetic efficiency and the difficulty of the circuit fabrication, it is preferable that the number of constituent magnets is set to a range of about 12 to 36.
As noted above, the constituent magnets 101 to 124, which are made of permanent magnets, are respectively magnetized with a specific period with respect to the radial direction, and seen from the central axis on the inner side, the constituent magnets corresponding to opposing poles are magnetized at an angular difference of 180°. Furthermore, adjacent constituent magnets are preferably magnetized at an angular difference as given in Equations (1) and (2). It should be noted that there may be changes of up to about ±5° in the magnetization directions, depending on usage conditions and optimization.
                              θ          ⁢                                          ⁢          n                =                              -                          360              N                                *          n                                    (                              n            =            1                    ,          2          ,                      …            ⁢                                                  ⁢                          N              /              2                                      )                            (        1        )                                          θ          ⁢                                          ⁢          n                =                  360          ⁢                      (                                          n                N                            -              1                        )                                              (                              n            =                                          N                /                2                            +              1                                ,                                    N              /              2                        +            2                    ,                      …            ⁢                                                  ⁢            N                          )                            (        2        )                θn: magnetization direction of n-th constituent magnet    N: total partition number (integer) of the magnetic field generating device    n: segment number (integer)
FIG. 8 is a schematic cross-sectional view of the dipole ring magnetic field generating device, taken at a plane perpendicular to the central axis. As shown in FIG. 8, when the Z axis is defined as the central axis of the magnetic field generating device, the Y axis is defined as the direction perpendicular to the Z-axis and substantially parallel to the unidirectional field generated in the space (on the radially inward side) within the ring (i.e. the NS magnetic field direction; direction of the main magnetic field component in FIG. 8), and the X axis is defined as the direction perpendicular to the Z-axis and the Y-axis (EW direction), then when the NS magnetic field direction (Y-axis direction) generated on the radially inward side of the dipole ring magnetic field generating device is taken to correspond to 0°, the angle of the magnetic field vector (also referred to as “skew angle” in the following) at any point on the radially inward side of the cylinder is substantially 0° at the radial center, due to the characteristics of the magnetic field generating device. On the other hand, an inclination of the skew angle can be observed that worsens (increases) when approaching the inner wall of the circuit.
When using an ordinary dipole ring magnetic field generating device, magnetic field components in which this skew angle is large often can be observed as impurities, that is, as noise. In particular the skew angle components in the plane (XY plane) on the radially inward side of the cylinder as shown in FIG. 8 seem to greatly affect the performance of elements manufactured with a manufacturing process of a substrate for semiconductors or the like, in particular with a process involving a heating step. For this reason the skew angle components need to be kept as small as possible.
In addition to the inner side of the dipole ring magnetic field generating device, these skew angle components are slower attenuated than the main magnetic field component on the outer side of the circuit, that is, at the aperture portions, so that the skew angle becomes larger on the outer side of the aperture portion. For this reason, when, in order to increase the throughput for example in view of mass production, a substrate that is still hot is retrieved to the outside of the circuit, then the skew angle components may cause considerable damage.
Moreover, Shin-Etsu Chemical Co. Ltd. has applied for a patent regarding a plasma processing device in which magnets are attached to the lateral wall of a dipole ring, for the purpose of reducing the magnetic field leaked from the dipole ring (see JP 10-041284A). However, even though it is possible to reduce the magnetic field leaked to the periphery, the reduction of the noise magnetic field in the extension of the radially inner space of the cylinder has been impossible.